1. Field
The presently disclosed embodiment concerns the field of bolted connections and more particularly bolted connections of large diameter, notably greater than M24, and high tightening torque, such as those used in the automotive, railway, maritime and even undersea, offshore, nuclear, etc. industries.
2. Brief Description of Related Developments
In these applications, a high level of safety and of reliability of the connection is required, which means in particular good reproducibility and repeatability of the residual tension after tightening. Also a certain ease and rapidity of use, even and above all in difficult environmental surroundings and under difficult environmental conditions, whether this be for reasons of accessibility, corrosion or nuclear radiation.
Three techniques are known for tightening bolted connections: torque tightening, thermal tightening, mechanical traction tightening.
Overall, the correct strength of a bolted connection is linked directly to the value of the residual tension in the connection after tightening.
The best known tightening method, which consists in tightening the connection by applying a torque to it by screwing action via the screwthread, is associated with a very wide dispersion of this residual tension. It is accepted that this dispersion is +/−20% in the best case scenario—with good control of the tightening torque—but is more generally ±/−60%.
What is more, torque tightening induces unwanted effects in the bolts, for example torsion effects, which render the connection even more uncertain.
Also, torque tightening implies friction between screwthreads, which gradually degrades the surfaces in contact and limits the possibility of effecting a plurality of successive tightenings/loosenings.
Finally, torque tightening generally does not make possible the simultaneous tightening of a plurality of connections, which further limits the fastening efficacy that it might be hoped to achieve.
Because of this, an alternative to torque tightening was invented a long time ago: tension tightening, with a first variant consisting in thermal tightening that employs thermal and/or differential expansion. However, given all the constraints associated with the use of heating to a high temperature, its use is limited.
Tension tightening is more simply effected by applying mechanical tension to the bolt.
Of these tensioning methods, of concern here is tensioning by hydraulic traction that makes it possible to control the residual tension in the shank by application of a pure traction force and makes a small dispersion possible.
In the field of hydraulic tensioning of threaded rods by pure traction, a plurality of methods exist.
There exists firstly a tightening method using a hydraulic tensioner. The tensioner is fitted to a standard bolt already mounted and pre-tightened or to a nut already in contact. The actuator stretches the bolt by hydraulic pressure and the nut is returned to contact with the parts to be clamped, which maintains the stretching of the bolt and produces the residual tension therein after relaxation.
The actuator is then removed, only the standard bolt remaining in place. The use of this type of technology also offers the possibility of tightening multiple connections simultaneously and therefore of obtaining a better distribution of the tensions in flanged connections.
There also exists a tightening method using a hydraulic nut. The hydraulic nut functions in accordance with the same principle as the hydraulic tensioner, the difference being that it replaces the standard nut on the bolt and therefore remains in place after tightening. Research has also been conducted into integrating a hydraulic piston into a bolt in order to stretch it. Placing the nut on the stretched bolt and releasing the pressure then produces the residual tension in the bolt.
In fact numerous devices, in particular hydraulic devices, are commercially available enabling tension tightening to be used, but it is clear that they may be complicated to use.
In fact this necessitates:
placing the bolt and the nut on the parts to be assembled;
placing the hydraulic tensioner and its hydraulic feed;
tensioning the bolt; locking the bolt;
verifying the tightening;
demounting the tensioner.
In an aggressive environment, for example a radioactive or deep water environment, or merely for economic reasons, it would be desirable to reduce these operating times, even to automate tightening and loosening when there is a requirement to assemble and to disassemble several times.